There is a requirement in industry for the measurement of conditions such as strain or temperature at all points over long distances. Typical uses are for monitoring oil and gas wells, long cables and pipelines. A typical wellbore has seepage of unwanted contaminants, especially in later stages of the production life of an oil or gas field. Temperature profiles along the wellbore can indicate where there are seepages or other problems and this can enable remedial action. Distributed temperature sensors often use Raman or Brillouin components of scattered light in optical fibres as the means to determine the temperature. Here, light from an optical source is launched into a fibre and the small amount of light that is scattered back towards the source is analysed. By using pulsed light and measuring the returning signal as a function of time, the backscattered light can be correlated to distance along the fibre. This backscattered light contains a component which is elastically scattered (Rayleigh light) and components that are up- and down-shifted in frequency from the source light (Raman and Brillouin anti-Stokes and Stokes light respectively, also known as inelastic scattered light). The powers of the returning Raman components are temperature dependent and so analysis of these components yields the temperature. The powers and frequency of the returning Brillouin components are strain and temperature dependent and so analysis of both components can yield temperature and strain independently.
The principles of analysing Brillouin backscatter for measuring strain and temperature has been described before, and reference is made to:    Parker, T. R., Farhadiroushan, M., Handerek, V. A., and Rogers, A. J., “Temperature and strain dependence of the power level and frequency of spontaneous Brillouin scattering in optical fibres”, Optics Letters, 1 Jun. 1997, Vol. 22, No. 11, pp. 787-789 and to:    Parker, T. R., Farhadiroushan, M., Feced, R., Handerek, V. A., Rogers, A. J., “Simultaneous Distributed Measurement of Strain and Temperature from Noise-Initiated Brillouin Scattering in Optical Fibres”, IEEE Journal of Quantum Electronics, Apr. 1998, Vol. 34, No. 4, pp. 645-659.
If the frequency and power of the Brillouin backscatter can be measured then the strain and temperature in the fibre can be determined.
It is known to install a suitable fibre in a conduit such as a pipe or bore at the time of commissioning, or installing or lining the pipe or bore. There are various known methods of protecting the fibre from damage and for fixing the fibre. Once the pipe or bore is in use, and carrying a fluid, typically pressurised, it is more difficult and costly to add a sensing fibre, or to deploy one temporarily and withdraw it after sensing. It is known to interrupt production from an oil well to perform what is known as a wellbore logging intervention. This can involve lowering logging tools into the wellbore, using gravity pull. When the wellbore angle/deviation become too high, normally above 70 degrees, a so-called “well tractor” can be used to overcome friction. This “tractor” will drive the tools into their desired location.
Another known method involves using coiled tubing, where the logging tools are brought to their desired location by pushing the coiled tubing from surface. A problem with coiled tubing is that it is subjected to buckling due to the nature of the coil being bent on a spool when deployed, even though straightening mechanism are used on the coil before this enters the wellbore. This can prevent the coiled tubing from reaching more than several hundred meters. This is not far enough into the wellbore for many applications.
Both these methods (“tractor” and coiled tubing) are expensive in terms of requiring a considerable number of personnel to operate, and in the case of coiled tubing, the surface systems are large and it can take a long time to rig up on the wellhead.
Also, to perform an intervention in a well using above methods, there is a need for vertical headroom above the wellhead. This may need to be 5 meters or more. In some locations, especially on offshore platforms, such vertical rig-up space is not available due to a deck or similar structure above.
It is known from U.S. Pat. No. 6,557,249 to Pruett that a process of inserting a fibre includes a step of strengthening an optical fibre by adding a UV curable resin around the fibre, to enable the fibre to resist higher temperatures in environments such as underground wellbores and to give it some stiffness. The strengthened fibre is passed through an impeller using wheels or a caterpillar track which pushes the fibre into a duct, optionally through a pressure chamber. The strengthened fibre is flexible enough to be bent round a 12 inch radius, and so could be liable to buckle if inserted into a large diameter pipe. The patent proposes insertion into a pre installed quarter inch capillary tube. This capillary tube would be pre installed within the wellbore, but this involves a separate process which adds considerable cost and complexity.
It is also known to insert fibres into conduits by fluid flow along the conduit. This clearly is not practical if the normal flow is against the direction of insertion. In some cases a separate narrower duct could be inserted into the conduit and the fibre floated into the duct, but this adds to the complexity and expense of the operation.